Doppler stated that waves reflected by an object have the same frequency as the waves incident thereon if the distance between the object and the source of the waves is constant, a higher frequency if the distance is decreasing, and a lower frequency if the distance is increasing. It is therefore possible to determine the velocity of an object with respect to the source by noting the difference between the frequencies of the waves emanating from the source and the waves reflected from the object.
This principle has been used to determine the respective velocities of a large number of particles, known in the art as "scatterers", that are moving in a stream flowing in a non-homogeneous medium, e.g., the respective velocities of particles of blood flowing in an artery, vein or heart chamber of a patient. In the usual apparatus for performing this function, pulses of pressure waves of a known frequency are launched by a transducer at a constant repetition rate into the medium along a path intersecting the stream. Energy contained in these pulses is reflected back to the transducer from locations along the path at which there is a change in acoustical impedance. Particles contained in blood generally constitute such variations in impedance. The transducer converts the reflected pressure waves into electrical waves, and means are provided for sampling the electrical waves at a time after the launching of said pulse at which the electrical waves relate to pressure waves reflected from scatterers of interest. Because the scatterers are moving, the amplitude of successive samples will change. Fourier analysis of this set of samples yields the respective amplitudes of a plurality of discrete frequencies. Each frequency corresponds to a different velocity, and its amplitude corresponds to the number of scatterers moving at that velocity. It is also possible to obtain an impression of the velocities of the scatterers by listening to a loudspeaker that is connected to the source of the samples with a suitable filter.
For reasons which are understood by those skilled in the art, the amplitude of each sample is proportional to the summation of reflections from scatterers contained in a sample volume having a length along the path equal to one-half of the product of the velocity of propagation of the wave and the duration of the pulse. Unfortunately, however, there are many situations where the length of the sample volume is not great enough to extend across the stream of scatterers for which the velocities are being sought, e.g., when the stream is flowing through one of the ventricles of a heart. This means that no indication will be attained of the velocities of scatterers along the path that do not lie within the sample volume. In order to include these scatterers in the sample volume, it has been customary to increase the duration of the pulses. However, where there is a limit to the power of the pulses that may be used as, for example, when pulses of pressure waves are being introduced into a patient's body for the purpose of measuring blood velocity, it is necessary to reduce the amplitude of the pulses when their duration is increased. It is also necessary to alter the receiver for each change in duration of the pulses if the signals are to be processed efficiently. The equipment required for these purposes is complex and expensive.